WDK

WDK (Wallet Development Kit) by Tether is an open-source toolkit for building multi-chain, self-custodial wallets. It handles key derivation, signing, and chain interactions across EVM, Bitcoin, Solana, Spark, and more. This guide covers using WDK with x402 as a buyer (paying for resources) and as a seller (accepting payments).

See a full working demo at github.com/SemanticPay/x402-usdt0-demo

Buyer

WalletAccountEvm satisfies the ClientEvmSigner interface that x402 expects. No adapter needed.

npm install @tetherto/wdk-wallet-evm @x402/fetch @x402/evm

import WalletManagerEvm from "@tetherto/wdk-wallet-evm";
import { x402Client, wrapFetchWithPayment } from "@x402/fetch";
import { registerExactEvmScheme } from "@x402/evm/exact/client";

const account = await new WalletManagerEvm(process.env.SEED_PHRASE, {
  provider: "https://rpc.plasma.to",
}).getAccount();

const client = new x402Client();
registerExactEvmScheme(client, { signer: account });

const fetchWithPayment = wrapFetchWithPayment(fetch, client);
const response = await fetchWithPayment("https://api.example.com/data");

Keys are derived locally from the seed phrase and never leave your environment. See the buyer quickstart for the full walkthrough.

Seller

Option 1: Hosted facilitator

Use Semantic’s facilitator to handle verification and settlement. Your server never interacts with the chain directly.

Automatic verification of buyer payment signatures
No gas fees, no native token required in your wallet
No transaction monitoring or retry logic
No facilitator infrastructure to deploy

Derive a receiving address from your seed phrase:

npm install @tetherto/wdk-wallet-evm @x402/express @x402/evm @x402/core

import WalletManagerEvm from "@tetherto/wdk-wallet-evm";

const account = await new WalletManagerEvm(process.env.SEED_PHRASE, {
  provider: "https://rpc.plasma.to",
}).getAccount();

const sellerAddress = await account.getAddress();

Then wire up Express with paymentMiddleware pointing to Semantic’s facilitator:

import express from "express";
import { paymentMiddleware, x402ResourceServer } from "@x402/express";
import { ExactEvmScheme } from "@x402/evm/exact/server";
import { HTTPFacilitatorClient } from "@x402/core/server";

const facilitatorClient = new HTTPFacilitatorClient({
  url: "https://x402.semanticpay.io/",
});

const app = express();

app.use(
  paymentMiddleware(
    {
      "GET /weather": {
        accepts: [
		{
            scheme: "exact",
            network: "eip155:9745", // Plasma mainnet
            price: {
              amount: "1000000", // $1.00 in base units (6 decimals)
              asset: "0xB8CE59FC3717ada4C02eaDF9682A9e934F625ebb", // USDT0 contract on Plasma
              extra: { name: "USDT0", version: "1", decimals: 6 }, // EIP-712 domain for signature
            },
            payTo: sellerAddress,
          },
        ],
        description: "Weather data",
        mimeType: "application/json",
      },
    },
    new x402ResourceServer(facilitatorClient).register(
      "eip155:9745",
      new ExactEvmScheme()
    )
  )
);

app.get("/weather", (req, res) => {
  res.json({ weather: "sunny", temperature: 70 });
});

app.listen(4021);

See the seller quickstart for the full walkthrough.

Option 2: Self-hosted facilitator

Run your own facilitator for full control over verification and settlement. The @semanticio/wdk-wallet-evm-x402-facilitator module wraps a WDK wallet as an x402 FacilitatorEvmSigner.

npm install @semanticio/wdk-wallet-evm-x402-facilitator @tetherto/wdk-wallet-evm @x402/core @x402/evm @x402/express

1. Create the facilitator signer

import WalletManagerEvm from "@tetherto/wdk-wallet-evm";
import WalletAccountEvmX402Facilitator from "@semanticio/wdk-wallet-evm-x402-facilitator";

const walletAccount = await new WalletManagerEvm(process.env.MNEMONIC, {
  provider: "https://rpc.plasma.to",
}).getAccount();

const evmSigner = new WalletAccountEvmX402Facilitator(walletAccount);

2. Initialize the facilitator

import { x402Facilitator } from "@x402/core/facilitator";
import { registerExactEvmScheme } from "@x402/evm/exact/facilitator";

const facilitator = new x402Facilitator()
  .onAfterVerify(async (ctx) => {
    console.log(`[verify] valid=${ctx.result?.isValid}`);
  })
  .onAfterSettle(async (ctx) => {
    console.log(`[settle] tx=${ctx.result?.transaction}`);
  });

registerExactEvmScheme(facilitator, {
  signer: evmSigner,
  networks: "eip155:9745",
});

3. Wire into Express

Same paymentMiddleware pattern as the hosted facilitator, but instead of an HTTPFacilitatorClient, pass the in-process facilitator directly to x402ResourceServer. Verification and settlement happen locally. For a complete working example, see server.js.

@semanticio/wdk-wallet-evm-x402-facilitator is currently in beta. Test thoroughly before using in production.

Summary

Role	Package	Adapter
Buyer	`@tetherto/wdk-wallet-evm`	None. `WalletAccountEvm` satisfies `ClientEvmSigner` directly.
Seller (hosted)	-	None. Use your address and point to Semantic’s facilitator.
Seller (self-hosted)	`@semanticio/wdk-wallet-evm-x402-facilitator`	Wraps `WalletAccountEvm` as `FacilitatorEvmSigner`.

Getting Started

Integration Guides

Buyer

Seller

Option 1: Hosted facilitator

Option 2: Self-hosted facilitator

1. Create the facilitator signer

2. Initialize the facilitator

3. Wire into Express

Summary

Next steps

​Buyer

​Seller

​Option 1: Hosted facilitator

​Option 2: Self-hosted facilitator

​1. Create the facilitator signer

​2. Initialize the facilitator

​3. Wire into Express

​Summary

​Next steps

Buyer

Seller

Option 1: Hosted facilitator

Option 2: Self-hosted facilitator

1. Create the facilitator signer

2. Initialize the facilitator

3. Wire into Express

Summary

Next steps